Palladium and palladium alloys have been traditionally used as contact surfaces for electrical contacts and connectors. Primarily, these alloys have been used in the form of wrought alloys or clad inlays as a replacement for gold in such applications. In recent years, many manufacturers of electrical contacts and connectors have been seeking methods to electroplate palladium or palladium alloys since, in many applications, electroplating would be more economical.
Many electrical contacts are manufactured by first electroplating a precious metal deposit in the form of a narrow band or stripe onto a wider strip or surface area of basis metal using high speed, reel-to-reel plating equipment. The electoplated strip is then stamped and formed into a contact with the precious metal electrodeposit located at the exact point where contact is to be made with the mating part. The electrodeposit on this formed part must be tightly adherent, sound, crack-free, and porosity-free, even after the stamping and forming operations. In order for an electrodeposit to withstand such operations, it must have sufficient ductility, good adhesion to the base metal, and freedom from porosity in the electroplated condition. Cracking of the electrodeposits cannot be tolerated in the final product. The electrodeposit should have sufficient ductility to withstand the stresses of stamping and forming without producing further cracks, pores or peeling from the substrate.
U.S. Pat. No. 4,269,671 discloses a method for electrodepositing a 60% by weight palladium 40% by weight silver alloy from a highly acidic solution containing a large amount of chloride ion. While the alloy obtained is a sound deposit, the plating solution is highly corrosive and causes severe displacement reactions to take place between the plating solution and the basis metal to be plated. These basis metals generally indude copper, nickel or their alloys. This type of high chloride plating solution for palladium/silver alloys as well known in the art as evidenced by Canadian Pat. No. 440,591. U.S. Pat. No. 4,269,671 discloses that the copper or nickel basis metals can be protected from the highly corrosive nature of such high chloride plating baths by first coating the basis metal with a thin layer of a precious metal. The precious metals suggested are silver and soft gold with the latter being preferred.
U.S. Pat. No. 4,463,060 describes a permanently solderable palladium/nickel electrodeposit of a thickness of about 0.1 to 1.5 micrometers having about 46 to 82 atomic percent palladium, balance nickel. This layer is covered by an extremely thin (i.e., about 20 angstroms) second layer of almost pure palladium. The second layer of palladium is formed not by electroplating, but by dipping the first layer into a solution of sulfuric or hydrochloric acid. This combination is described as forming a permanently solderable palladium/nickel electrodeposit.